Space separated tags in `DD_TAGS` are not included in DogStatsD

[hfsaito]

Other packages, gem and the heroku buildpacks are using DD_TAGS with space separated format.
Checking TagSerializer and seems it only expects to have comma separated tags in DD_TAGS which prevents from correctly loading tags from environment variable.

dogstatsd-ruby/lib/datadog/statsd/serialization/tag_serializer.rb

Lines 81 to 85 in bf701af

	def dd_tags(env = ENV)
	return {} unless dd_tags = env['DD_TAGS']
	to_tags_hash(dd_tags.split(','))
	end

And this only runs in StatsD initialization, so it can be tricky to patch this section since needs to be patched before initialization

[nightpool]

See documentation here:
https://docs.datadoghq.com/getting_started/tagging/assigning_tags

After installing the containerized Datadog Agent, you can set your host tags using the environment variable DD_TAGS in your Agent’s main configuration file. If you specify multiple tags, separate each one with a space.

[hfsaito]

The documentation for heroku-buildpack-datadog

DD_TAGS

Optional. Sets additional tags provided as a space-separated string (Note: comma-separated string in buildpack versions 1.16 and earlier; this is still supported to maintain backward compatibility). For example, heroku config:set DD_TAGS="simple-tag-0 tag-key-1:tag-value-1". The buildpack automatically adds the tags dyno which represent the dyno name, such as web.1, and dynotype (the type of dyno, e.g run or web). See the Guide to tagging for more information.

https://docs.datadoghq.com/agent/supported_platforms/heroku/#configuration